In the recording method by means of an inkjet printer which is one of typical methods among various color recording methods, various methods for discharging ink have been developed, and they are all a method where ink droplets are generated and adhered onto various record-receiving materials (such as paper, film and cloth) to perform recording. This recording method has been rapidly prevailing lately and is expected to continue growing remarkably in the future because of features such as quietness without noise generation due to no direct contact of a recording head with a record-receiving material and as easiness in downsizing, speedup and colorization.
Conventionally, as an ink for fountain pens, felt-tip pens or the like and as an ink for inkjet recording, water-based inks where a water-soluble dye is dissolved in an aqueous medium have been used, and in these water-based inks, a water-soluble organic solvent is generally added to prevent ink from clogging at a pen tip or an inkjet nozzle. These conventional inks are required to provide recorded images with sufficient density, not to clog at a pen tip or a nozzle, to dry quickly on a record-receiving material, to bleed less, to have excellent storage stability and so on. In addition, recorded images formed are required to have fastnesses such as water fastness, light fastness and moisture fastness.
Meanwhile, images or character information on color displays of computers are generally expressed by subtractive color mixing of 4 color inks of yellow (Y), magenta (M), cyan (C) and black (K) for color recording by an ink jet printer. In order to reproduce, as faithfully as possible, hue of an image expressed by additive color mixing of red (R), green (G) and blue (B) on CRT displays and the like, through images by subtractive color mixing, it is desired that each of Y, M and C has a hue as close to each standard as possible and also vividness. In addition, it is required that ink compositions to be used for them are stable in storage for a long period of time, and that images printed therewith have a high concentration and are excellent in fastnesses such as water fastness, light fastness, and gas fastness.
The application of inkjet printers has been widely spread in the fields ranging from small printers for office automation to large printers for industrial use, and therefore fastnesses such as water fastness, moisture fastness, light fastness, gas fastness and the like have been required more than ever. Water fastness has been largely improved by coating organic or inorganic particulates of porous silica, cation polymer, aluminasol, special ceramic and the like together with a PVA resin on a paper surface to provide an image receiving layer on a record-receiving material. “Moisture fastness” means durability against a phenomenon of bleeding of the coloring matter in a record-receiving material when the colored record-receiving material is stored under an atmosphere of high humidity. Bleeding of coloring matter extremely deteriorates image quality in images particularly required to have high resolution, photo-like image quality and therefore it is important to reduce such bleeding as far as possible. As for light fastness, technique for large improvement thereof has not established yet. In particular, many of original coloring matters for magenta among 4 primary colors of Y, M, C and K have low light fastness and therefore improvement thereof is an important problem. In addition, there are more opportunities to print pictures at home with recent spread of digital cameras, and image discoloration by oxidizing gases such as ozone gas, nitrogen oxides and the like in the air where printed matters obtained are stored is acknowledged as a problem. Oxidizing gas has a nature to react with the coloring matter on or in a recorded paper to cause discoloration or fading of the printed image. Among oxidizing gasses, ozone gas is regarded as a main causative agent accelerating color-fading of inkjet-recorded images. This phenomenon of discoloration or fading is characteristic of inkjet images, and therefore improvement of ozone gas fastness is an important challenge as well as improvement of light fastness.
“Water fastness” means durability against a phenomenon of bleeding or fading of the coloring matter in a record-receiving material in the case where water droplets dropped on the colored record-receiving material are wiped off, and therefore improvement thereof is one of important problems to solve. As for silver salt photographs, for example, even when water is carelessly poured on the surface of a photo image, the quality of the image is not particularly deteriorated if the water is wiped off.
However as for inkjet recording images, extreme deterioration of the recorded image due to bleeding or the like of the coloring matter easily occurs in the same situation though it has photo image quality, and in order to prevent this, it cannot help using a special method such as coating of the recorded image with, for example, a resin.
At present, examples of such the special method include, for example, the lamination method, which is however not so spread among ordinary users at home and therefore it is desired that water fastness of recorded image is improved by improvement of coloring matter, ink composition and the like.
As a coloring matter-skeleton of magenta coloring matters used in water-soluble inks for inkjet recording, typical are xanthene coloring matters and azo coloring matters using H acid (1-amino-8-hydroxy-naphthalene-3,6-disulfonic acid). However, the former is very excellent in hue and vividness but very inferior in light fastness. On the other hand, some of the latter are good in terms of hue but inferior in light fastness, gas fastness and vividness. As for this type, a magenta dye excellent in vividness and light fastness has been developed but it still has a low level in light fastness compared with other dyes having other hues such as a cyan dye represented by a copper phthalocyanine coloring matter and a yellow dye.
Examples of such a magenta coloring matter excellent in vividness and light fastness also include an anthrapyridone coloring matter. Specifically, Patent Literature 1 discloses a mixture containing a sodium salt of formula (1) described later and an ink composition using the mixture. However, the study of the present inventors found that by using said mixture, vividness and light fastness are considerably improved but water fastness is extremely inferior and thus the fastnesses are not satisfying the above requirements. In addition, Patent Literature 2 discloses a structural formula of a trisodium salt of a compound represented by formula (1) described later as “Coloring matter 1” and 1H-NMR thereof in heavy water and discloses discoloration or fading tests to an ink using “Coloring matter 1” and to printed samples using said ink, and discloses that images having high fastnesses can be obtained. With respect to fastnesses, however, there is a study on only color fading by light and there is no description on other various fastnesses as described above. Further, the 1H-NMR values, in heavy water, of the trisodium salt described in said Patent Literature 2 is obviously different from 1H-NMR values, in heavy water, of the trisodium salt of the compound represented by formula (1) described later which is synthesized by the inventors of the present invention. Furthermore, Patent Literature 2 does not disclose the synthesis method and therefore it cannot be confirmed what said compound is actually like and how much various fastnesses required above are improved.    Patent Literature 1: JP H10-306221 A (pp. 1-17)    Patent Literature 2: JP 2006-124611 A (p. 11)